1. Field of the Invention
The present invention relates to fuel cell systems and driving methods therefor, and more specifically, to a fuel cell system including a secondary battery which is electrically connected with a fuel cell, and a method of starting the system.
2. Description of the Related Art
Fuel cells take time until they attain a temperature appropriate for power generation, after being started at an ambient temperature. While the temperature is low, the power generation output of the fuel cell is low. When starting, therefore, fuel cell systems obtain energy from power supplies other than the fuel cell in order to drive their system components, etc. Fuel cell systems cannot start themselves without an energy supply other than the fuel cell. Further, even if a fuel cell system has an energy supply such as a secondary battery, a problem will be encountered during a start-up of the fuel cell system if there is not sufficient supply of energy from the secondary battery until the fuel cell has attained a temperature appropriate for sufficient power generation.
A fuel cell system which includes a secondary battery is disclosed in JP-A 9-231991, for example. JP-A 9-231991 discloses a technique for supplying a load with electric power from a secondary battery when the system is started, during which warming-up state of the fuel cell is monitored. When it is determined that the fuel cell is warmed up to a sufficient level, the fuel cell is connected with the load so that the fuel cell supplies electric power to the load.
However, the fuel cell system according to JP-A 9-231991 does not monitor the amount of energy stored in the secondary battery, and therefore can fail to start the fuel cell system if sufficient power is not stored in the secondary battery.